Glass preform with deep radial gradient layer and method of manufacturing same

ABSTRACT

A glass preform structure, preferably of an optical glass, constructed with a central core of a first material, a surrounding tube of a second material, and a deeply placed bonded layer integrally formed between the core and tube preferably by a heat driven interdiffusion of the first and second materials. The deeply placed interface layer of the resulting preform structure exhibits material characteristics related to the interdiffused material characteristics of the rod and tube materials. The interdiffusion is preferably performed while supporting the combined rod and tube structure. The preform is rotated during heating to maintain the geometric symmetry of the preform and of the interface layer. An encapsulating carrier is used to support the preform in all dimensions during heating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to imaging and non-imagingoptical preforms utilized, for example, in the fabrication of opticalfibers, emitters, and sensors and, in particular, to the formation of aunique optical preform having a deeply placed radially bonded interfacelayer of controlled radial depth and symmetry.

2. Description of the Related Art

There are numerous applications and methods of producing opticalpreforms. Some of the more common preform applications include use asthe source component for the drawing of optical fiber, as bulk sourcematerial for lens blanks, and as the cap or encapsulating lens ofoptical emitters. In these and other uses, the optical, mechanical andthermal properties of the optical preform and the precise definition ofthese properties is greatly valued. Furthermore, a graded variation ofthese properties within the finished product, either through structuralor material processing, is also greatly desired.

At least three primary methods of fabricating optical preforms relevantto the present invention are conventionally known. The first is the useof chemical vapor deposition (CVD) to deposit a material on the interiorsurface of a glass tube. The object of this process is to provide a coreportion of a material having a first set of optical characteristicssurrounded by a cladding layer having a second set of opticalcharacteristics. Creation of the optical preform requires the use of acladding tube formed from a high purity silica based glass, typicallycomposed of greater than 95% silica, with a small amount of a lowdiffusivity dopant added to establish the optical characteristics of thecladding. The low diffusivity of the dopant is required to minimizethermal migration if not direct loss of the dopant during processing. Ahigh purity silica vapor, though also containing a low concentration ofa selected dopant, is then pumped through the cladding tube while thecladding is heated in a zone that is mechanically moved repeatedly alongthe length of the cladding to facilitate the actual deposition of silicaand dopant from the vapor phase onto the interior surface of thecladding. Selection of the vapor phase dopant material and itsconcentration, and thereby the optical characteristics of the corematerial formed by deposition, is particularly limited by therequirement of uniform deposition of the dopant relative to thedeposited silica. In addition, temperatures and flow rates must befurther carefully maintained to achieve the uniform deposition of thesilica while retaining a uniform dopant concentration in the originaland deposited material. Precise temperature control is also required soas not to overheat the cladding, resulting in asymmetrical deformationthat would, in turn, compromise the desired geometric structure of theoptical preform. Once a layer of the core silica material has beendeposited, a high temperature treatment must be uniformly applied to thecladding and core to collapse the entire structure as necessary to fillthe center of the preform.

The CVD process is not only costly and complex due to the requiredprecision at many process steps, but the process is quite time intensivesince the rate of uniform vapor phase deposition is inherently low.Perhaps the most significant limitation, however, is that substantialmaterials limitations are present due to the fundamental nature of theprocess. In particular, the cladding and vapor deposited core materialsare required to be of the same elemental glass composition,conventionally referred to as being of the same glass "family." Examplesof conventional glass families include borosilicate glasses, leadglasses, and barium glasses. By virtue of the core and cladding being ofthe same glass family and the ratio of dopants to silica in both beingquite low, the difference in material properties between the core andcladding is inherently limited. For example, CVD preforms aresubstantially limited to a core to cladding difference in index ofrefraction of about 0.1 and more typically 0.03 or less.

Also, the thermal and mechanical properties of the vapor deposited coreand cladding materials are highly interdependent in order to performcorrectly in the final collapse stage of the process so that unduestrain is not placed on the cladding material. Consequently, theoptical, mechanical and thermal properties of the preform fabricated ina CVD process are significantly limited.

Another process for forming optical preforms uses ion diffusion to alterthe surface optical, mechanical and thermal properties of an otherwisehomogeneous optical material rod. In this process, the rod is placed inan ion salt bath and heated to a temperature sufficient to encourage iontransport at the surface of the rod material. In effect, a leaching ofthe surface material occurs resulting in an alteration of the materialproperties within the leached zone. This zone can be formed to asubstantial radial depth, though only at the surface of the rodmaterial. As a practical matter, however, the zone can achieve a radialdepth of only a fraction of a millimeter to several millimeters during aleaching period of about three to four months. Furthermore, due to iontransport mechanics being highly dependent on the specific ionconcentration at the surface of the optical rod material, precisecontrol of the resulting optical characteristics is quite difficult. Theleaching action also directly reduces the material strength andintegrity of the preform in the affected zone. Consequently, thefinished optical properties of the resulting preform may vary to adegree that is not commercially acceptable for many optical preformuses.

Finally, a third method for forming an optical preform is to simplycollapse a cladding tube of an optical material onto a rod of the sameor different optical material. U.S. Pat. No. 4,486,214, issued to Lynchon Dec. 4, 1984, discloses an example of this process. The object of thepreform fabrication process disclosed there is to create a preformhaving a sharply defined change in index of refraction between the innerrod and outer tube material. Although not as limited as in CVDprocesses, the choice of materials for the outer tube is limited by therequirement that the outer tube collapse uniformly onto the rod withoutdeformation of the rod material. The thermal and mechanicalcharacteristics of the collapsing tube must therefore be chosen to allowa uniform collapse at a temperature appropriately below the meltingtemperature of the rod material. The collapsed tube is ultimately fusedto the rod material not as part of the formation of the preform, butonly during a separate subsequent step of drawing the optical preforminto optical fiber. The simultaneous step of fusing and drawing thefiber allows the abrupt optical interface between the tube and rodmaterial to be sharply maintained, the precise goal of preforms made bythis process. However, materials fusion at the drawing stage alsomaintains any impurities and gaps that may exist at the materialinterface. Consequently, a premium is placed upon the initial precisecollapse of the tube on the rod in performance of this process.

SUMMARY OF THE INVENTION

A general purpose of the present invention is therefore to provide anoptical preform that, in the preform form, achieves a deeply placedbonded interface layer that represents an interface layer of controlledthickness with a physical or optical gradient related to the propertiesof a center core material and an outer cladding material.

This is achieved by a preform structure constructed with a central rodof a first material, a surrounding tube of a second material, and ashallow or deep interface layer integrally formed between the rod andtube, preferably by a heat driven interdiffusion of the first and secondmaterials. The interface layer of the resulting preform structureexhibits material characteristics related to the interdiffused materialcharacteristics of the rod and tube materials.

A process of creation of the preform structure includes the steps of (1)collapsing a tube onto a rod, (2) supporting the combined structure, and(3) heating the preform structure to form the deeply placed bonded layerat the interface between the tube and the rod. The preform is preferablyrotated about its cylindrical axis during heating. An encapsulatingcarrier is preferably used to support the preform during heating androtation in the formation of deep interface layers.

Thus, an advantage of the present invention is that it is capable ofachieving a continuous or smooth stepped radially graded property regionin an optical preform structure. The gradient profile of the region isdirectly related to and significantly controlled by the radial depth ofthe region.

Another advantage of the present invention is that it efficientlyprovides a highly controllable deeply placed bonded radial interfacelayer within an optical preform providing for a smooth transition of oneor more properties between two material layers. The controlledproperties may include but are not limited to index of refraction,thermal expansion coefficient, density, material composition, tensileand compressive strength, and thermal transfer properties.

A further advantage of the present invention is that the deeply placedradial bonded interface layer may achieve a radial thickness or depth ofup to the full radial dimension of the optical preform. Furthermore, theinterface layer may effectively consume either the rod or tube layer,depending substantially on the relative radial dimensions of the rod andtube.

Yet another advantage of the present invention is that the process formanufacturing the inventive optical preform structure is not limited toany particular family of materials, such as lead silica, borosilica,barium silica or the like.

Still another advantage of the present invention is that any number ofconcentric deeply placed interface layers, each having a differentradius, can be constructed in accordance with the present invention.

Yet still another advantage of the present invention is that the processfor manufacturing the inventive optical preform maintains the structuraland optical symmetry of the preform.

A still further advantage of the present invention is the use of athermal gravitational control step for affecting the thermal migrationof dopant atoms in the deeply placed radial bonded interface layer and,thereby, the gradient of material properties radially through the deeplyplaced radial bonded interface layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages and features of the present invention willbecome better understood upon consideration of the following detaileddescription of the invention when considered in connection of theaccompanying drawings, in which like reference numerals designate likeparts throughout the figures thereof, and wherein:

FIG. 1 is a illustrative perspective view of a tube shown prepared forbeing collapsed about a center rod;

FIG. 2a is a representative end view of a collapsed rod and tubestructure and a graph showing the variation of a material property, suchas index of refraction, having an abrupt variance with respect to depth;

FIG. 2b shows an end view of a collapsed rod and tube structure with anintervening deeply placed radial bonded interface layer in accordancewith a preferred embodiment of the present invention and a graphrelating a material property, such as index of refraction, having asmooth stepped radial gradient across the deeply placed radial bondedinterface layer as a function of radial depth;

FIG. 2c shows an end view of a multiple collapsed tube and rod preformstructure in accordance with a preferred embodiment of the presentinvention with two deeply placed radial bonded interface layersinterposed between successive tube layers, and a graph showing a smoothstepped radial gradient of a material property, such as index ofrefraction, varying with radial depth;

FIG. 2d is a graph showing a smoothly stepped radial gradient of amaterial property, such as index of refraction, extending across thefull depth of the preform structure;

FIG. 3a is an illustrative perspective view of a preferred ceramicroller mechanism for rotating an optical preform structure constructedin accordance with the present invention for purposes of annealing andcreation of a deeply placed radial bonded interface layer;

FIG. 3b is an illustrative perspective view of the preferred ceramicroller mechanism with a compressive roller for eliminating the presenceof trapped gas and voids at the interface between the collapsed tube androd; and

FIG. 4 is an illustrative perspective view of a glass carrier utilizedto encapsulate an optical preform structure consistent with a designrequirement to permit rotation in creating a deeply placed radial bondedinterface layer in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The initial stage in fabricating an optical preform consistent with thepresent invention is shown in FIG. 1. While the process for completingthe fabrication of the optical preform will be described in detail, itshould be readily apparent to those skilled in the art that many changesand modifications, particularly in terms of dimensions and choice ofmaterials, can be made without departing from the invention in itsbroader aspects. Accordingly, the specific embodiments disclosed areonly representative in providing a basis for the claims which define thescope of the present invention.

In FIG. 1, an optical preform 10 is shown in the preferred process ofconstruction from an initial cylindrical rod 12 inserted through amatching cylindrical hollow in a tube 14. The material of the rod 12 andtube 14 may be chosen from any of a number of families of opticalmaterials, typically referred to as glassy materials, including leadsilica, borosilica, and other glass families that are well known in theart. Typically, each family of glasses is characterized as a glasshaving the same constituent components, though delimited by the generalrequirement of maintaining an adequate crystalinity, clarity andelasticity for the glass.

In accordance with the present invention, the rod 12 and tube 14 neednot be formed from similar materials or even of materials from the sameglass families. Rather, the process of manufacturing the optical preform10 is compatible with rod and tube materials that are quite dissimilarin optical, mechanical and thermal characteristics as defined by choiceof differing material compositions of the glasses. However, selection ofthe materials for the rod 12 and tube 14 are not without limitation. Inaccordance with the present invention, the materials must be selected soas to have generally similar melting temperatures such that duringsubsequent processing, the predominant mechanism for the mixing ofmaterials between the rod 12 and tube 14 is interdiffusion rather thanconvection mixing. Generally, a melting temperature difference of withinabout 100 degrees Celsius and preferably within about 50 degrees Celsiusis adequate for purposes of the present invention. Also, the choice ofglass components of the rod 12 and tube 14, when interdiffused, mustproduce a glass having an adequate crystalinity, clarity and elasticity.

Another limitation is that the coefficients of expansion between the rod12 and tube 14 must be relatively closely matched to permit processingof the optical preform 10 to temperatures at or above the softeningpoints of the rod and tube glassy materials and with controlled slowcooling back to room temperature. Naturally, the mechanical propertiesof both the rod 12 and tube 14 materials must be considered indetermining whether the coefficient of expansion are adequately matchedfor the particular materials. Specifically, where the coefficient ofexpansion is greater for the rod material than for the tube, thebrittleness of the tube 14 will likely serve as the limiting factor indetermining whether the coefficients of expansion are sufficientlymatched to preclude cracking of the tube 14 during cooling.

The separate choice of materials must also permit a rather finelypolished surface to be created on the exterior cylindrical surface ofthe rod 12 and interior bore surface of the tube 14. In order to createa uniform structure, voids of all kind must be avoided at the interfacebetween the rod 12 and tube 14. In order to insert the rod into the boreof the tube 14, the radius of the rod 12 must be slightly less than thatof the bore. As the optical preform structure 10 is heated, any gaspresent must not be impeded in flowing out of the annular gap betweenthe rod 12 and tube 14. Even where all gas is evacuated prior toheating, the creation of voids may occur in the presence of rough matingsurfaces. Thus, even in evacuated manufacture of the preform 10,polished mating surfaces are required. The degree of polishing, however,depends greatly upon the materials utilized, but can be readilydetermined to be sufficient where microscopic examination of the opticalpreform reveals the absence of voids above a diameter acceptable for theintended use of the optical preform. In general, an adequate degree ofpolishing is not difficult to obtain.

Finally, the dimensions of the annular spacing between the rod 12 andtube 14 must be sized to account for any greater coefficient of thermalexpansion by the rod 12 relative to the tube 14, particularly where themelting point of the rod material is lower than that of the tubematerial. Ideally, the annular space is sized to permit the rod materialto reach its melting point with a radial expansion sufficient to closethe annular space between the rod 12 and the tube 14 without placing anexpansive force on the interior surface of the tube 14 in excess of theexpansive strength of the tube 14.

Referring now to FIG. 2a, an end view of the preform 10 followingcollapse of the tube 14 onto the rod 12 is shown. While index ofrefraction is illustrated, all of the material properties of the preform10 are essentially uniform within the rod 12 and tube 14, respectively.By the creation of a deeply placed radial bonded layer at the interfacebetween the rod 12 and tube 14, as shown in FIG. 2b, an interface layer16 is constructed with material properties that vary through a smooth,continuous step from those of the rod 12 to those of the tube 14.Furthermore, this deeply placed bonded interface layer 16 is intimatelyfused with and formed from an intermix of both the material of the rod12 and of the tube 14.

As generally indicated in FIG. 2c, multiple concentric deeply placedbonded interface layers can be created both concurrently as well assuccessively. A second optical preform 20 is shown with a central rod22, a first tube 26 cylindrically surrounding the rod 22, and a secondtube 30 cylindrically surrounding the first tube 26. For concurrentcreation of interface layers 24, 28, the tubes 26, 30 are both collapsedonto the rod 22 before beginning a process of creating of the deeplyplaced bonded interface layers 24, 28.

For sequential generation of the interface layers 24, 28, the first tube26 may be collapsed upon the rod 22 and the composite preform structureprocessed to form a deeply placed bonded interface layer 24 having aninitial desired thickness. That is, the composite preform 22, 26 may beheated in accordance with the present invention so as to fuse or bondthe mating surfaces of the tube 26 and rod 24, thereby creating at leastan initial thickness of the interface layer 24. A second tube 30 maythen be collapsed upon the outer surface of the tube 26 and theresulting composite preform structure again heated in accordance withthe present invention to complete the formation of the interface layer24 and to create a second interface layer 28, both to their finalintended thicknesses. As generally indicated by the variance in index ofrefraction between the various layers, the interface layers 24, 28establish regions of continuous smooth stepped properties between thecentral rod 22, inner tube 26, and outer tube 30.

Further in accordance with the present invention, the heating of thecomposite structures shown in FIG. 2a, 2b and 2c can be performed to anextent where there is an interdiffusion of the rod and tube materialsthrough the full depth of the rod and tubes. Accordingly, asrepresentatively shown in FIG. 2d, interdiffusion of the rod and tubematerials into one another can proceed to a point where there is asymmetrical single smooth step radial gradient extending across the fulldepth of the composite preform 22, 26, 30.

Referring now to FIG. 3a, the initial thermal processing of a compositepreform 32 from a simple rod and tube structure will be described. Oncea glass rod 34 has been mechanically inserted into the cylindrical boreof a tube 36, the composite structure 32 is placed within a furnace (notshown) that is substantially conventional in nature. The relevantmodification to the furnace is the provision for ceramic rollers 38provided within the interior of the furnace to support and provide forthe rotation of the composite preform 32. In the preferred embodiment,with a preform having a diameter of approximately one inch, each of therollers 38 have an approximate diameter of 20 millimeters.

At this stage of processing, the goal is to heat the composite preform32 to a temperature close to but generally above the softeningtemperature of the tube material 36 while allowing the rod material toexpand and fuse to the interior bore surface of the tube 36. Rotation ofthe composite assembly 32 during this thermal processing results in theformation of a uniform bonded interface layer joining the rod 34 to thetube 36. Rotation of the composite assembly 32 also enhances theuniformity of heating across the entire length of the composite assembly32.

Preferably, the temperature of the composite preform 32 is heated at aconventionally determinable rate, related to the thermal shockresistance of the selected materials, from an initial ambienttemperature to a temperature just above the softening temperature of thetube 36; a "near" softening temperature. At about the temperature wherethe rod will have expanded to contact the inner bore surface of the tube36, the rotation of the ceramic rollers 38 is initiated to obtain arotation rate of between about 50 and 150 revolutions per minute of thecomposite preform 32, though much higher rotation rates may prove to beadvantageous and lower rates adequately effective.

Once the near softening temperature of the tube 36 has been reached, anadditional ceramic roller 38', as generally shown in FIG. 3b, is pressedagainst the outer surface of the tube 36 in mechanical opposition to theceramic rollers 38. The pressure roller 38' preferably presents a smallcontact surface to the exterior surface of the preform 32. In thepreferred embodiment, the pressure roller 38' has a diameter ofapproximately 20 millimeters and an approximate contact width of twomillimeters. The rotation of the rollers 38, 38' and the preform 32 ismaintained during the pressing, with the contact surface of the pressureroller 38' being translated along the length of the preform 32. Theforce applied by the roller 38' is selected to be sufficient tosubstantially drive out any remaining atmospheric gas and voids that maybe present at the interface between the rod 34 and tube 36. The ceramicroller 38' is then removed and the composite assembly 32 is maintainedrotating at the near softening temperature of the tube 36 for a periodof time sufficient to fuse the outer surface of the rod 34 to the innerbore surface of the tube 36.

A controlled cooling of the composite preform 32 is then performed, withthe reduction in temperature again being at a conventionallydeterminable rate. Once the temperature of the rod 34 and tube 36 hasdecreased to a point intermediate between the near softening temperatureof the tube 36 and ambient, characterized as at least where there is nofurther significant interdiffusion activity between the rod and tubematerials, the rotation of the ceramic rollers 38 may be halted.

An annealing of the rod 34, tube 36 and interface between the rod 34 andtube 36 is then preferably allowed to occur at the intermediatetemperature level. This annealing step is intended to reduce thematerial stresses created at the interface between the rod 34 and tube36, as well as in the rod 34 and tube 36 as a consequence of thermalprocessing. This annealing step is performed for a period of time thatis conventionally determinable based on the materials used, the sizesand geometries of the components, and the bonding temperature and time.After the annealing step is complete, the composite preform 32 may thenbe slowly cooled to ambient temperature.

The resulting deeply placed bond between the rod 34 and tube 36represents an intimate fusing of the rod and tube materials across athickness that is directly related to the relative interdiffusioncharacteristics of the materials making up the rod 34 and tube 36, thetemperature at which the bond formation occurs, the length of time thatthe fusing is continued, and, to a lesser degree, the rate of rotationof the composite preform 32.

A glass carrier 40 as used in the further processing of the compositepreform 32 is shown in FIG. 4. The glass carrier 40 is constructedpreferably of an outer high temperature resistant quartz glass tube 42having one blunt closed end and a second quartz tube 44 of similarmaterial also having one blunt closed end. The composite preform 32,coated with a non-stick agent such as finely powered boron nitride isplaced in and adjacent the closed end of the outer tube 42. While othernon-stick agents may be used, boron nitride powder is preferred for itshigh melting temperature even in the presence of an oxidizingatmosphere.

Preferably, the outer diameter of the composite preform 32 and the innerdiameter of the tube 42 are chosen to closely match, with allowance fora thin continuous layer of the non-stick agent. The outer diameter ofthe second tube 44 is also preferably sized to match the inner diameterof the first tube 42. The second tube 44 is inserted into the first tube42 so as to abut the end of the composite preform 32 with the bluntclosed end of the tube 44. The total cylindrical length of the tubes 42,44 is preferably selected such that the open ends opposite the preform32 can be heat sealed without conducting sufficient heat to the preformcompartment formed in the carrier 40 to heat the composite preform 32 byany appreciable portion of the softening temperature of the preformmaterials.

The inner and outer tubes 44, 42 need not be sealed along the entiretyof their mutual cylindrical interface 46. Rather, a slight annularinterface region may be allowed to remain between the outer cylindricalsurface of the inner tube 44 and the cylindrical bore surface of theouter tube 42. In order to allow venting of gases from within thisannular region during subsequent heat processing of the carrier 40 andpreform 32, a pinhole passage 48 is provided between this annular regionand the open interior of the second tube 44.

While the forgoing relation between the inner and outer tubes 44,42 isthe preferred configuration, the high temperature processing of thepreform 32 also requires that there is minimal leakage of liquifiedglass material from the preform compartment. Where the materials of thepreform and non-stick agent have been selected such that there is littleoutgassing from the preform compartment, the annular region between theinner and outer tubes 44,42 may be sealed and the pin hole 48 omitted.

The function of the glass carrier 40 is to allow processing of thecomposite preform 32 at temperatures above the softening temperature ofboth the rod and tube material. This high temperature processing isperformed by placing the carrier 40, including the composite preform 32,into the furnace and onto the ceramic rollers 38. The temperature withinthe furnace is preferably raised at a conventionally determinable rateto a temperature in excess of the softening temperature of both the rod34 and tube 36. This higher temperature is selected to maximize the rateof interdiffusion of the rod and tube materials by interdiffusion, butwithout any substantial degree of convective mixing. In order tomaximize this allowed temperature, minimize the likelihood of convectivemixing due to uneven heating of the preform 32, and to maintain thestructural symmetry of the preform 32, the carrier 40 is rotatedbeginning at a temperature significantly below the softening temperatureof either the rod or tube material. The rate of rotation is preferablysufficient to allow the preform 32 to be uniformly heated within thefurnace.

The rotation of the carrier 40 may permit the present invention to applya preferential bias to the relative rate of interdiffusion of the rodmaterial into the tube material. The magnitude of this bias will dependgreatly on the densities of the glasses and densities of the dopantspresent within the glasses. In any event, a modest rate of rotation willinsure that the interdiffusion between the rod and tube materialsremains symmetrical. Higher rates of rotation may result in theinterdiffusion of rod material into the tube material at a greater rateand to a greater depth than the diffusion of tube material into the rodmaterial. Since the optical, thermal and mechanical properties of theresulting composite preform 32 will depend on the relativeconcentrations of the rod and tube materials particularly in the deeplyplaced interface layer formed between the rod and tube 34, 36 the rateof rotation may be used to directly impact the properties achieved.

The rotation of the carrier 40 is maintained for a period that willsubstantially define the thickness of the interface layer formed betweenthe rod 34 and tube 36. Interface layer thicknesses of up to about 0.1millimeter can be referred to as shallow. Interface layer thicknesses ofgreater than about 0.1 millimeter can be referred to as deep. Thepresent invention is readily capable of the controlled production ofshallow interface layers only several atom diameters thick to deepinterface layers tens of millimeters thick limited only by the radius ofthe composite preform itself.

After formation of the interface layers, a controlled cooling of thecarrier 40 is performed down to an annealing temperature below thesoftening temperatures of both the rod and tube material as well as thatof the deep bonded layer. This annealing temperature is, however,preferably selected to be sufficient to anneal the deeply placedinterface layer. Rotation of the carrier 40 by the ceramic rollers 38 ispreferably halted once this annealing temperature is reached. After aconventionally determinable annealing time, the carrier 40 is cooledslowly to a final ambient temperature.

The carrier 40 then may be cracked open to yield the completed compositepreform 32. The resulting preform 32, once cleaned in a conventionalmanner, can be further processed into an optical fibers or opticallenses having unique properties characterized by a precisely controlledand deeply placed radial gradient.

EXAMPLES

Fabrication of preforms, first to a stage where a fused shallow bondexists and then to a stage where a deep bonded interface layer has beencreated, is described in the Examples below. Both stages are achievedfor two different preform material combinations. The first two examplesuse glassy materials from the same glass family. The second two examplesdemonstrate use with glassy materials from two different glasscomposition families.

Example 1: Deeply Placed Shallow Interface Bond with Custom Lead Glasses

An optical preform was fabricated from two cylindrical pieces of glass.A solid core, or central rod, was approximately 12.78 millimeters indiameter. The cladding tube had an outside diameter of approximately25.25 millimeters and an inside diameter of approximately 12.90millimeters. Both components had a length of approximately 28millimeters. The core and cladding were fabricated from custom leadglasses of substantially matching composition. The principle differencebetween the two compositions was a slightly different ratio of silica tolead, accounting for a difference in the index of refraction. The corehad an index of refraction of 1.67 and a softening temperature ofapproximately 630 degrees Celsius. The cladding had an index ofrefraction of 1.59 and a softening temperature of approximately 665degrees Celsius.

The core was prepared by core drilling the approximate outside diameterfrom a glass slug of the core material. The final polished diameter wasachieved by allowing the core to rotate on a rapidly revolving diamondimpregnated pad, first at 350 mesh and then at 1250 mesh.

The cladding sample was prepared from a glass slug of the claddingmaterial by first core drilling the inside diameter and then coredrilling the outside diameter to form a cylinder. The inner cylindricalbore surface was then polished to its final diameter by applying arapidly rotating diamond impregnate pad against the surface, first witha 350 mesh and then with a 1250 mesh.

The annular space between the core and the cladding was chosen, in viewof the above material characteristics of the core and cladding, tocompensate for the anticipated differential expansion during heating.

The core was inserted in the cladding cylinder and this assembly wasplaced on the rotation roller mechanism within the furnace. Inaccordance with the present invention, it is desired to apply both heatand rotation to the mechanically prepared core and cladding assembly inorder to realize the initial fusion bonding of the core and claddingmaterials. A conventional furnace was modified to accept ceramic rollersextending completely through the furnace. The ceramic rollers wereexternally supported and driven. The core and cladding assembly waspositioned freely upon the rollers within the furnace.

The core and cladding assembly was heated to an interface bondingtemperature of 670 degrees Celsius over a period of three hours.Rotation was started once the temperature reached 560 degrees Celsius;rotation was then maintained at a rate of approximately 97 revolutionsper minute. Once at 670 degrees Celsius, the assembly was slowly pressedfrom end to end using a ceramic roller, having an approximately 20millimeter diameter and an approximately 2 millimeter wide contactsurface, pressed against the core and cladding assembly for a period ofbetween about 15 and 60 seconds. The rolling axis of the pressure rollerwas slightly skewed relative to the rolling axis of the preform. Therotation of both the pressure roller and the preform were maintainedsubstantially synchronous throughout the pressing. This pressing wasdone to ensure that any residual air bubbles were driven to the ends ofthe assembly. During the pressing the furnace incidentally cooled to 620degrees Celsius. Once the pressing was completed, the temperaturereturned to 670 degrees Celsius for the remaining duration of theinterface bonding period.

At the conclusion of the interface bonding period, an annealing step wasperformed. The interface bonded preform, the furnace and rotationmechanism were cooled from the bonding temperature to 440 degreesCelsius at a rate of about 2.0 degrees Celsius per minute. Once reaching450 degrees Celsius, rotation was halted. The temperature was then heldat 440 degrees Celsius for one hour and then cooled to 360 degreesCelsius at about 0.2 degrees Celsius per minute. The furnace was thenallowed to slowly cool to room temperature over a period of severalhours.

After reaching room temperature, the diameter of the interface bondedpreform was measured to be approximately 25.10 millimeters. A smallslice was taken off one of the ends for optical measurement. Thediameter of the interface bond was measured to be approximately 0.062millimeters.

This process was successful in creating a deeply placed shallowinterface bond of controlled and uniform thickness, formed in intimatefused contact with the core and cladding layers, and consisting of acombination of the core and cladding materials.

A materials stress analysis, performed by placing polarizing filters oneach axial end of the preform slice, determined that there was alocalized region of stress closely surrounding the core, but no apparentstress in the core itself. This localized stress pattern is generallydesirable, since compressive stress about the core adds strength to thepreform.

Example 2: Deeply Placed Deep Bonding Interface Layer with Custom LeadGlasses

This Example 2 utilized the shallow interface bonded preform prepared inExample 1. In order to maintain the form and geometry of the preformatthe higher temperatures necessary for the formation of the deepinterface layer, the preform was supported in all dimensions by a hightemperature resistant quartz glass encasement having internal dimensionsclosely matching the external dimensions of the preform. Prior toinsertion into the encasement, the preform was first completely coatedwith fine powdered boron nitride as a non-stick agent. After insertion,the encasement was sealed off, leaving only a small air hole to allowthe escape of gasses. The prepared preform and encasement were thenreturned to the furnace and placed on the roller assembly.

The preform and encasement were then heated to 840 degrees Celsius overa total period of approximately four hours and 20 minutes. Rotation ofthe preform and encasement was initiated once the temperature reached595 degrees Celsius; the rotation was at a rate of approximately 82revolutions per minute. Once the temperature reached 840 degreesCelsius, the temperature was held constant for approximately four hours.Thereafter, the preform and encasement were cooled to 440 degreesCelsius at 3.81 degrees Celsius per minute. The preform and encasementwere held at 440 degrees Celsius for one hour and the rotation stopped.The preform and encasement were then cooled to 320 degrees Celsius atapproximately 0.21 degrees Celsius per minute, and then allowed toslowly cool to room temperature. Once at room temperature, theencasement was cracked open and the deep interface layer preform wasreadily released.

The dimensions of the preform at the end of the deep bonding processmeasured approximately 24.89 millimeters in diameter. The thickness ofthe deep bonded layer was measured at approximately 1.31 millimeters.This deep interface layer was found to exist uniformly between the coreand cladding layers.

A stress analysis of a center 11.28 millimeter length of the deep bondedlayer preform revealed localized compressive stress patterns surroundingthe core, again indicating a generally desirable increase in thestrength of the preform.

Example 3: Deeply Placed Shallow Interface Bond with Specialty Glasses

The core and cladding were prepared substantially as described inExample 1, though differing as follows. The solid core was approximately81.20 millimeters long and 12.70 millimeters in diameter. The claddinghad length of approximately 80.95 millimeters with an outside diameterof approximately 25.40 millimeters and an inside diameter ofapproximately 12.98 millimeters.

The core material was a lead borosilicate glass with an index ofrefraction of 1.56, a density of about 3 grams per cubic centimeter, anda softening temperature of about 600 degrees Celsius. The cladding wasan alkali borosilicate glass with an index of refraction of 1.48, adensity of 2.4 grams per cubic centimeter, and a softening temperatureof approximately 628 degrees Celsius. Both the core and claddingmaterials are commercially available compositions obtainable fromSpecialty Glass Corporation, 305 Marlborough Street, Oldsmar, Fla.,34677.

The core and cladding assembly was heated to the bonding temperature of660 degrees Celsius in a period of three hours. Rotation, at a rate ofapproximately 96 revolutions per minute, was started when thetemperature reached 600 degrees Celsius. Once at 660 degrees Celsius,the assembly was pressed from one end to the other using theapproximately 20 millimeter ceramic roller, while both continued to berotated. During the pressing, the furnace incidentally cooled to 630degrees Celsius. Following the pressing, the furnace was reheated to 660degrees Celsius, and held for ten minutes. A second, otherwise identicalpressing was then performed.

To anneal the preform, the furnace and rotation mechanism were cooledfrom the bonding temperature to 460 degrees Celsius at a rate of about5.7 degrees Celsius per minute. Upon reaching 460 degrees Celsius,rotation was stopped. The temperature was held at 460 degrees Celsiusfor one hour and then cooled to 383 degrees Celsius at about 0.9 degreesCelsius per minute. The furnace was then allowed to slowly cool to roomtemperature over a period of several hours.

After reaching room temperature, the preform diameter was measured to beapproximately 24.94 millimeters. A small slice was taken off one of theends for measurement. The thickness of the interface bond layer wasmeasured to have a thickness of approximately 0.035 millimeters.

Again, a deeply placed shallow interface bond of controlled and uniformthickness was achieved. Also, the compressive stress region localizedaround the core of the preform was again found to exist, indicating thatthe preform had increased in strength.

Example 4: Deeply Placed Deep Bonding Interface Layer with SpecialtyGlasses

The shallow interface bond preform prepared in Example 3 was furtherprocessed substantially as described in Example 2, though differing asfollows.

A center slice of about 25.26 millimeters in length was cut from theshallow interface bonded preform. After placement of the preform in theencasement, the preform and encasement were heated to 760 degreesCelsius over a period of three hours and ten minutes. Rotation, at arate of about 83 revolutions per minute, was started when thetemperature reached 460 degrees Celsius. Once the temperature reached760 degrees Celsius, the temperature was held for two hours. Theencasement and preform were then cooled to 460 degrees Celsius atapproximately 3.5 degrees Celsius per minute, then held at 460 degreesCelsius for one hour. Rotation was then stopped and the encasement andpreform were cooled to 350 degrees Celsius at approximately 0.2 degreesCelsius per minute. The encasement and preform were then allowed toslowly cool to room temperature.

The diameter of the deep interface layer preform, at the end of theprocess, was approximately 25.12 millimeters. The thickness of the deepbonded layer was measured to have a thickness of 0.45 millimeters; atleast a full order of magnitude greater than the thickness of thestarting shallow interface layer. Also, the compressive stress regionlocalized around the core of the preform was again found to exist,indicating an increase in strength.

From the forgoing Examples it can be seen that the present invention mayreadily utilize materials of quite different optical, mechanical andthermal properties, subject to the limitations described above.Specifically, by choice of materials from different glass families, suchas a lead glass for a core rod and a barium or rare earth glass for thetube, an index of refraction difference as high as 0.3 or higher can beachieved with the present invention.

Thus, unique preform structures, characterized by highly controlleddeeply placed shallow and deep interface layers, and a method ofproducing the same has been described. The foregoing description of thestructures and method, as well as the identification of the applicablematerials and properties is intended to be illustrative rather thanexhaustive. Accordingly, as will be readily appreciated by apractitioner of skill in the relevant arts, other materials andproperties are within the scope of the appended claims.

We claim:
 1. A glass fiber preform, suitable for use in the productionof lens and fibers, comprising:a) a core of a first glass material, saidcore having a central axis; b) a tube of a second glass materialsurrounding said core along a length of said central axis; c) a deeplyplaced bonded interface layer formed between said core and said tube bya radially driven interdiffusion of said first and second materials,wherein said first and second glass materials have respective first andsecond predetermined material properties and wherein said deeply placedbonded interface layer has a material property that is a product of saidradially driven interdiffusion of said first and second materials,wherein said tube has a thickness perpendicular to said central axis ofgreater than about one millimeter, and wherein said material property ofsaid deeply placed bonded interface layer varies along a smooth gradientbetween said core and said tube.
 2. A glass preform formed by a processcomprising the steps of:a) providing a glass tube on a glass core toform an initial preform structure; b) supporting said initial preformstructure to substantially maintain the shape and finite size of saidinitial preform structure; and c) heating said initial preform structureto form a deeply placed radial bonded layer having a uniform radialgradient at the interface between said tube and said core, whereby saidinitial preform structure becomes said glass preform, wherein said stepof supporting provides for substantially maintaining the finitedimensions of said initial preform structure through said step ofheating.
 3. The glass preform of claim 2 wherein said process furthercomprises the step of rotating said initial preform structure on an axisparallel to a predetermined axis of said initial preform structureduring said step of heating.
 4. The glass preform of claim 3 wherein,prior to said step of heating, said process further comprises the stepsof:a) coating the external surface of said initial preform structurewith a non-stick agent; and b) placing said initial preform structure ina heat resistant case structurally conforming to the external surface ofsaid initial preform structure.
 5. A glass preform comprising:a) auniform central cylindrical core of a first optical material; b) auniform covering cylindrical layer concentrically surrounding saidcentral cylindrical core, said covering cylindrical layer being of asecond optical material; and c) a deeply placed bonded interface layerconcentrically interposed between said central cylindrical core and saidcovering cylindrical layer, wherein said deeply placed bonded interfacelayer is of an optical material consisting of said first opticalmaterial and said second optical material in proportion, along a radialgradient, to the mutual inter-diffusion characteristics of said firstand second optical materials, wherein the radial mid-point of saiddeeply placed bonded interface layer, relative to the cylindrical axisof said optical preform, is at a depth of greater than about onemillimeter from the radially distal surface of said covering cylindricallayer.
 6. The optical preform of claim 5 wherein said deeply placedbonded interface layer extends radially across substantially the entireradius of said fiber preform.
 7. A preform of finite length having aglass structure with a fixed uniform cylindrical surface contour and apredetermined material characteristic that varies according to apredetermined radial profile consistently for all radiuses of said glassstructure, said preform being formed by a process comprising the stepsof:a) providing one or more glass layers of a softened glass materialover a primary surface of a substantially unsoftened glass core materialto form an integral glass structure, each said glass layer having arespective reproducibly uniform thickness and substantially reproducingthe surface contour of said primary surface; b) heating said glasslayers and glass core material to cause mutual interdiffusionthereinbetween; and c) rotating said integral glass structure about anaxis substantially parallel to said primary surface during said step ofheating to effect reproducible uniform mutual interdiffusion betweensaid glass layers and said glass core material.
 8. The preform of claim7 wherein said predetermined material characteristic is the index ofrefraction.
 9. The preform of claim 7 wherein said predetermined radialprofile is defined by the mutual interdiffusion of one or more glasslayers, each having a predetermined radial thickness that is uniform andconsistently reproducible, circumferentially overlying a central glasscore.
 10. The preform of claim 9 wherein said predetermined materialcharacteristic is the index of refraction and wherein said glass corecomprises a first initial glass material having a first predeterminedindex of refraction and wherein a predetermined one of said glass layersis comprised of a second initial glass material having a secondpredetermined index of refraction.
 11. The preform of claim 7 whereinthe process of forming said preform further comprises the step ofsupporting said integral glass structure so as to maintain thereproduction of the surface contour of said primary surface through saidstep of heating and wherein said step of heating provides for theheating of said integral glass structure to a temperature greater thanabout the softening temperature of said glass core material.
 12. Thepreform of claim 11 wherein, in the process of forming said preform,said step of heating provides for the mutual interdiffusion between saidglass layers and said glass core material to extend substantiallythrough the thickness of a predetermined one of said glass layers. 13.The preform of claim 12 wherein there are two or more glass layers.